Electron beam tubes



Aug. 7, 1956 R. w. PETER ELECTRON BEAM TUBES Filed June 2, 1952 [NI 'E\"I'OR. RUL W. PETER Uaited States Patent 2,758,243 Patented Aug. 7,1956 Fice ELECTRON BEAM TUBES Rolf W. Peter, Princeton, N. J., assignorto Radio Corporation of America, a corporation of Delaware ApplicationJune 2, 1952, Serial No. 291,225

Claims. (Cl. 315--3.5)

This invention relates to electron beam tubes, and particularly, tohelix type traveling wave amplifier tubes.

The object of the invention is to provide improved means for couplingthe circuit helix of a traveling wave tube to the input and outputtransmission lines.

In the drawing:

Fig. l is a schematic view of a conventional helix type traveling wavetube; and

Fig. 2 is an axial sectional view of a helix type traveling wave tubeembodying the invention.

In a conventional traveling wave amplifier tube of the helix type, anelectron beam is projected along an elongated helix at a velocityapproximately equal to the axial phase velocity of waves traveling alongthe helix. By successive processes of electron velocity modulation andinductive output, the beam is modulated by the electric fields set upalong the helix by the signal wave, and the modulated beam gives upenergy to the wave to increase its amplitude.

Suitable means are provided at the input and output ends of the helixfor coupling the helix to input and output transmission lines. Thesetransmission lines may be waveguides or coaxial lines. Fig. 1 shows aconventional tube in which the input end of the helix 1 is coupledthrough the dielectric envelope 4 to an external input waveguide 3extending transversely of the tube axis by means of an axially-extendedvariable-pitch transition portion 1a of the helix extending parallel tothe electric field component E1 in the waveguide 3. A conventionalelectron gun G, including a cathode 5, focusing electrode 7 andaccelerating electrode 9, is mounted in an enlarged end portion 4a ofthe dielectric envelope 4, in axial alignment with the helix 1. Thedistance from the accelerating electrode 9 to the end of thevariable-pitch transition portion 1a, where the active portion of thehelix begins, is indicated by the letter L1. The opposite end of thehelix 1 is coupled through the envelope 4 to an external outputwaveguide 11 by means of an axiallyextended variable-pitch transitionportion 1b of the helix, similar to transition portion 1a, extendingparallel to the electric field component E2 in the waveguide 11. Acollector electrode 13 is mounted adjacent to the end of the transitionportion 16. The distance from the end of the variable-pitch portion 111to the collector 13 is indicated by L2. The active length of the helixis indicated by La.

It is of greatest importance to make a traveling wave tube as short aspossible for a given gain. No axial space should be used by anythingthat has no importance for beam forming and beam-field interaction. Inconventional designs considerable axial space is used up to provide abroad-band match between the feeding lines and the helix, as illustratedin Fig. 1.

The conventional coupling arrangement shown in Fig. 1 has severaldisadvantages. First, the parts of the electron beam that extend alongthe two transition portions 1a and 1b are not electrically shielded fromthe surrounding dielectric envelope wall, and hence, variable potentialsare produced along the inner wall of these parts of the envelope duringoperation. These variable potentials cause undesirable variations inbeam velocity along the tube axis in these regions. A seconddisadvantage of the arrangement of Fig. 1 lies in the increased cost ofproviding a suitable axial focusing magnetic field for preventingspreading of the beam and consequent collection of electrons by thehelix. The gain of a traveling wave amplifier tube is proportional tothe active length of the helix. However, the focusing magnetic fieldmust be provided over substantially the entire beam path, as indicatedby the arrows H in Fig. 1. Hence, any reduction in the overall length ofthe beam path, such as by reducing the distances L1 and L2, results inlower cost for a tube of given active length, or gain. Third, therelatively large distance L1 may impose a serious limitation on thesignal-to-noise ratio of the tube. In order to obtain lowest noisefigure, it may be necessary that the distance L1 be as small aspossible. The shorter the active helix, the larger the relative lengthof the coupling antenna and the more important to eliminate the latter.

In accordance with the present invention, the input end of the activehelix is positioned close to the accelerating electrode of the electrongun and the helix is inductively coupled to the external inputtransmission line by means of a conductor located entirely within thetube envelope and extending laterally from the end of the helix and backalong the helix away from the electron gun. In the case of waveguideinput, the helix extends at least partly through the input waveguide toa point close to the electron gun and the coupling conductor extendsback across the waveguide substantially parallel with the high frequencyelectric field therein. Furthermore, the output end of the active helixis positioned close to the collector electrode and the helix isinductively coupled to the output transmission line by means of asimilar conductor located entirely within the envelope and extendinglaterally from the end of the helix and back along the helix away fromthe collector electrode and toward the input coupling conductor. Inwaveguide output, the helix extends at least partly through the outputwaveguide and the coupling conductor extends back across the waveguidesubstantially parallel with the high frequency electric field therein.The helix itself does not couple to the electric fields Within thewaveguides. Therefore, these fields are not disturbed by the helixrunning across them.

As shown in Fig. 2, the active helix 1 extends across the input andoutput waveguides 3 and 11 to points close to the accelerating electrode9 and collector electrode 13, respectively. The ends of the helix 1 areinductively coupled to the fields within the input and output waveguidesby conductors 15 and 17 which extend laterally from the ends of thehelix and then back along the helix toward each other in directionssubstantially parallel with the electric fields E1 and E2, respectively,within the two waveguides. In the arrangement shown in Fig. 2, theelectric fields E1 and E2 and the coupling conductors 15 and 17 areparallel with the axis of the helix. The coupling conductors 15 and 17may be either integral extensions of the helix wire, as shown in Fig. 2,or separate conductors connected to the helix ends. The conductors 15and 17 are located entirely within the envelope of the tube 4.

In Fig. 2 is shown a magnet structure suitable for use with waveguidetype input and output lines for focusing the electron beam. Thisstructure comprises two pole pieces 19 and 21 connected by one or morepermanent magnet rods or bars 23. The various electrodes of the tube areprovided with suitable supporting means and external leads for applyingsuitable direct-current potentials. These have not been shown inthe'd'rawing since they are not part of the present invention.

The present invention provides a traveling wave tube in which each ofthe distances L1 and L2 is reduced to a small fraction of thecorresponding distance in the conventional tube of Fig. 1, so that theoverall beam path is not much longer than the active helix length L3.

The arrangement shown does not involve any mechanical connection betweenthe traveling Wave tube and the surrounding waveguides or magnetstructure. Therefore, the tube is readily insertable and removable fromvarious waveguide and magnet structures.

Instead of projecting the electron beam through the interior of thehelix of the tube, as shown, an annular electron gun may be provided toproject a hollow beam along the outside of the helix, in which case thecoupling conductor or antenna would be folded back inside the helixinstead of outside the latter.

What is claimed is:

l. A traveling wave electron tube comprising an elongated dielectricenvelope containing an elongated conductive helix, an electron gun closeto one end of said helix for projecting a stream of electrons along saidhelix for interaction with waves traveling thereon, and means forinductively coupling said end of said helix through said envelope to anexternal input transmission line, said means including a couplingconductor located entirely within said envelope and extending laterallyfrom said end and back over said helix away from said electron gun.

2. A traveling wave electron tube comprising an elongated dielectricenvelope containing an elongated conductive helix, means for projectinga stream of electrons along said helix for interaction with wavestraveling thereon, means for coupling an. external input transmissionline to the end of said helix adjacent to said projecting means, acollector electrode in the path of said stream close to the opposite endof said helix, and means for inductively coupling said opposite endthrough said envelope to an external output transmission line, saidlastnamed means including a coupling conductor located entirely withinsaid envelope and extending laterally from said opposite end and backover said helix away from said collector electrode.

3. A traveling wave electron tube comprising an elongated dielectricenvelope containing an elongated conductive helix, an electron gun closeto one end of said helix for projecting a stream of electrons along saidhelix for interaction with waves traveling thereon, a collectorelectrode in the path 'of said stream close to the opposite end of saidhelix, and means for inductively coupling the two ends of said helixthrough said envelope to external input and output transmission lines,said means including a coupling conductor located entirely within saidenvelope and extending laterally from each of said ends and back oversaid helix toward each other.

4. A traveling wave electron tube comprising an elongated dielectricenvelope containing an elongated conductive helix and an electron gunclose to one end of said helix for projecting a stream of electronsalong said helix for interaction with waves traveling thereon, and meansfor coupling an external high frequency signal transmission line to saidend of said helix, said means including a pair of conductors external tosaid envelope and axially spaced along said helix adjacent said endthereof to form an axial gap and adapted to be coupled to said line toestablish a high frequency electric field across said gap and parallelto the axis of said helix, and a coupling conductor located entirelywithin said envelope and extending laterally from said end and backalong said helix parallel to said axis to couple inductively throughsaid envelope with said field.

5. A traveling wave tube comprising an elongated dielectric envelopecontaining an elongated conductive helix, a section of waveguide ofrectangular cross section external to and extending transversely of saidenvelope and having a pair of apertures in opposite Walls thereofthrough which said envelope extends, said helix extending through atleast one of said apertures and having one end positioned close to andin alignment with the other aperture, a coupling conductor locatedentirely within said envelope and extending laterally from said end andback over said helix away from said other aperture for inductivelycoupling through said envelope with the transverse high frequencyelectric field within said waveguide, and an electron gun in saidenvelope close to said helix for projecting a stream of electrons alongsaid helix for interaction with waves traveling thereon.

References Cited in the file of this patent UNITED STATES PATENTS Re.21,739 Llewellyn Mar. 4, 1941 2,238,770 Blumlein Apr. 15, 1941 2,575,383Field Nov. 20, 1951 2,584,802 Hansell Feb. 5, 1952 2,623,129 Lerbs Dec.23, 1952

